elf: Accept absolute (SHN_ABS) symbols whose value is zero [BZ #23307]

We have this condition in `check_match' (in elf/dl-lookup.c):

  if (__glibc_unlikely ((sym->st_value == 0 /* No value.  */
                         && stt != STT_TLS)
                        || ELF_MACHINE_SYM_NO_MATCH (sym)
                        || (type_class & (sym->st_shndx == SHN_UNDEF))))
    return NULL;

which causes all !STT_TLS symbols whose value is zero to be silently
ignored in lookup.  This may make sense for regular symbols, however not
for absolute (SHN_ABS) ones, where zero is like any value, there's no
special meaning attached to it.

Consequently legitimate programs fail, for example taking the
`elf/tst-absolute-sym' test case, substituting 0 for 0x55aa in
`elf/tst-absolute-sym-lib.lds' and then trying to run the resulting
program we get this:

$ .../elf/tst-absolute-sym
.../elf/tst-absolute-sym: symbol lookup error: .../elf/tst-absolute-sym-lib.so: undefined symbol: absolute
$

even though the symbol clearly is there:

$ readelf --dyn-syms .../elf/tst-absolute-sym-lib.so | grep '\babsolute\b'
     7: 00000000     0 NOTYPE  GLOBAL DEFAULT  ABS absolute
$

The check for the zero value has been there since forever or commit
d66e34cd4234/08162fa88891 ("Implemented runtime dynamic linker to
support ELF shared libraries.") dating back to May 2nd 1995, and the
problem triggers regardless of commit e7feec374c63 ("elf: Correct
absolute (SHN_ABS) symbol run-time calculation [BZ #19818]") being
present or not.

Fix the issue then, by permitting `sym->st_value' to be 0 for SHN_ABS
symbols in lookup.

	[BZ #23307]
	* elf/dl-lookup.c (check_match): Do not reject a symbol whose
	`st_value' is 0 if `st_shndx' is SHN_ABS.
	* elf/tst-absolute-zero.c: New file.
	* elf/tst-absolute-zero-lib.c: New file.
	* elf/tst-absolute-zero-lib.lds: New file.
	* elf/Makefile (tests): Add `tst-absolute-zero'.
	(modules-names): Add `tst-absolute-zero-lib'.
	(LDLIBS-tst-absolute-zero-lib.so): New variable.
	($(objpfx)tst-absolute-zero-lib.so): New dependency.
	($(objpfx)tst-absolute-zero: New dependency.

1 files changed, 6 insertions, 2 deletions

diff --git a/elf/Makefile b/elf/Makefile
index 0eb7c8114e..41cc3681be 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -186,7 +186,7 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-tlsalign tst-tlsalign-extern tst-nodelete-opened \
 	 tst-nodelete2 tst-audit11 tst-audit12 tst-dlsym-error tst-noload \
 	 tst-latepthread tst-tls-manydynamic tst-nodelete-dlclose \
-	 tst-debug1 tst-main1 tst-absolute-sym tst-big-note
+	 tst-debug1 tst-main1 tst-absolute-sym tst-absolute-zero tst-big-note
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -273,7 +273,7 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		tst-latepthreadmod $(tst-tls-many-dynamic-modules) \
 		tst-nodelete-dlclose-dso tst-nodelete-dlclose-plugin \
 		tst-main1mod tst-libc_dlvsym-dso tst-absolute-sym-lib \
-		tst-big-note-lib
+		tst-absolute-zero-lib tst-big-note-lib
 
 ifeq (yes,$(have-mtls-dialect-gnu2))
 tests += tst-gnu2-tls1
@@ -1467,6 +1467,10 @@ LDLIBS-tst-absolute-sym-lib.so = tst-absolute-sym-lib.lds
 $(objpfx)tst-absolute-sym-lib.so: $(LDLIBS-tst-absolute-sym-lib.so)
 $(objpfx)tst-absolute-sym: $(objpfx)tst-absolute-sym-lib.so
 
+LDLIBS-tst-absolute-zero-lib.so = tst-absolute-zero-lib.lds
+$(objpfx)tst-absolute-zero-lib.so: $(LDLIBS-tst-absolute-zero-lib.so)
+$(objpfx)tst-absolute-zero: $(objpfx)tst-absolute-zero-lib.so
+
 # Both the main program and the DSO for tst-libc_dlvsym need to link
 # against libdl.
 $(objpfx)tst-libc_dlvsym: $(libdl)